[swift-evolution] Ad hoc enums / options

[Leonardo Pessoa]

I think I should start writing that proposal for enum case names now.

L

-----Original Message-----
From: "Vladimir.S via swift-evolution" <swift-evolution at swift.org>
Sent: ‎01/‎06/‎2016 07:02 PM
To: "Tony Allevato" <allevato at google.com>
Cc: "swift-evolution" <swift-evolution at swift.org>
Subject: Re: [swift-evolution] Ad hoc enums / options

 > in other words, we could consider allowing this:
 >    func foo(bar: (.fit | .fill)) {
 >      baz(bar: bar)
 >    }
 >    func baz(bar: (.fit | .fill | .florp) { ... }
 >
 > In other words, an ad hoc enum T can be used wherever an ad hoc enum U is
 > expected if T ⊆ U.

Can't agree with this. Just because the same analogue with tuples : 
differently defined tuples are different types. Tuples with different order 
of types in declaration - are different types. So I expect here instance of 
(.fit | .fill) `bar` is not of the same type as (.fit | .fill | .florp)

But +1 to be able to 'convert' instance of (.fit | .fill) to instance of 
(.fit | .fill | .florp). For example(if we'll have init(caseName) and 
.caseName for enums):

func foo(bar: (.fit | .fill)) {
     let bazbar = (.fit | .fill | .florp).init(caseName: bar.caseName)
     baz(bar: bazbar)
}
func baz(bar: (.fit | .fill | .florp) { ... }



On 02.06.2016 0:38, Tony Allevato wrote:
> I find myself agreeing with the idea that ad hoc enums are to enums as
> structs are to tuples. Based on that analogy, why should an ad hoc enum
> *need* a name (autogenerated or otherwise) any more than a tuple needs a
> name? Would those who dislike ad hoc enums argue that this also shouldn't
> be allowed:
>
>     func foo(bar: (x: Int, y: Int)) {}
>     let t: (x: Int, y: Int) = (x: 5, y: 5)
>
> If someone writes `(.fit | .fill)` (or whatever the hypothetical syntax
> might be), that should just *be* the type the same way that `(x: Int, y:
> Int)` is a type without a name, and that type can be used in argument
> lists, variables, or whatever. There shouldn't be any worry about
> declarations across multiple functions colliding or being incompatible any
> more than we would worry about two functions declaring arguments of type
> `(x: Int, y: Int)` would collide or be incompatible.
>
> One side of ad hoc enums that I'd like to see explored is that, by being
> unnamed, they're basically anonymous finite sets and we could apply
> well-defined subset relationships to them: in other words, we could
> consider allowing this:
>
>     func foo(bar: (.fit | .fill)) {
>       baz(bar: bar)
>     }
>     func baz(bar: (.fit | .fill | .florp) { ... }
>
> In other words, an ad hoc enum T can be used wherever an ad hoc enum U is
> expected if T ⊆ U.
>
>
> On Wed, Jun 1, 2016 at 1:43 PM L. Mihalkovic via swift-evolution
> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>
>
>
>     > On Jun 1, 2016, at 6:51 PM, Vladimir.S <svabox at gmail.com
>     <mailto:svabox at gmail.com>> wrote:
>     >
>     > Yes, I also can support the idea of autogenerated type name (like
>     Enum_fit_OR_fill) as long as it allows to do all the things we are
>     discussing here: declare (.fit|.fill) in function, use .fit on calling
>     side, use (.fit|.fill) to declare temporary variable of type compatible
>     with such function parameter etc.
>     >
>
>     It all works because the compiler is just being a thoughless scribe
>     that just writes the standard enum we don't bother to write ourselves.
>     Because the heuristic is simple and straightforward then it is
>     predictible. The enum can be used with its long name be ause it is a
>     real enum. And writing the short form of it also works because the
>     compiler knowns uniquely what the long name is everytime it runs into
>     the short name.
>
>
>     > But how do you suggest to define a type of such function in
>     `typealias` for example? i.e. for func my(option: (.fit|.fill) {..}
>     >
>     > typealias MyFunc = ((.fit|.fill)) -> ()
>     > or as
>     >
>     > typealias MyFunc = (Enum_fit_OR_fill) -> ()
>     >
>
>     Ideally there is no difference whatsoever, there is a single enum, it
>     is produced at the module level, and it has the long form name.
>
>     There can be rules that would prevent us from doing that with
>     externally visible APIs, if the core team fuges that we should take the
>     time to write our enums manually and cleanly to make them visible to
>     the world, but it is not a necessary rule.
>
>
>     >
>     > But I still can't support the idea of limiting the usage of such
>     enums - i.e. "To deal with milti site definition, the compiler would
>     simply flag a error/warning, or be silent in the presence of a new
>     annotation:". I really think we need then introduce the same rule for
>     tuples - so no one can use the same tuple declaration in function -
>     they then should declare separate struct type or use @something for
>     such functions. Nobody wants such rule for tuples.
>     >
>
>     Multi site thing is not a limitation... Is is a proposed rule to say
>     that we are able to be lazy twice without being penalized. Yhe compiler
>     does not like when we define the same thing twice, and thse short form
>     amount to doing what he does not let us do. But because this is about
>     concise and lazy, then the compiler can let us get away with it if we
>     use an annotation that lets it know that "it is not a mistake.. I
>     really dont want to write that enum myself, even though I am using the
>     same abbreviation twice". Otherwise, the compiler would let us know
>     that the second time could be a mistake because there is already
>     something with the same name...
>
>     But again this is a separate idea from the core notion of a syntax
>     sugaring for writing real enums the lazy (clever) way
>
>     >> On 01.06.2016 19:04, L. Mihalkovic wrote:
>     >> The only problem with this proposal is to consider them ad-hoc
>     enums... If we view them as having nothing ad-hoc about them and the
>     thing to be a simple sugaring exercise, then I think all the opositions
>     on grounds of breaking the language disapear. It still does not mean it
>     should be done if the core team does not like the idea of encouraging
>     laziness, or simply do not like what it makes them look like. No matter
>     what, this type of sugaring exercise has been clearly stated as out of
>     scope for 3.0
>     >>
>     >>>> On Jun 1, 2016, at 2:38 PM, Vladimir.S via swift-evolution
>     <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>     >>>>
>     >>>> On 01.06.2016 11:00, Austin Zheng wrote:
>     >>>> Tuples are a structural type, they are described entirely by the fact
>     >>>> that they are a tuple, plus their contained types.
>     >>>>
>     >>>> Enum cases are not individual types; that precedent exists nowhere in
>     >>>> Swift. You can't (yet) build a structural type out of something that
>     >>>> isn't a type. The fact that you had to propose something like
>     >>>> "AdhocEnumFitFill_2383748" as an autogenerated name for the type
>     >>>> demonstrates the proposal's weaknesses: a tuple is an ad-hoc type that
>     >>>> describes itself, while an anonymous enum isn't.
>     >>>
>     >>> Yes, I understand the point about the type of such adhoc enum.
>     >>> The only workaround I can see in this case(if we'd really want to
>     have it in language) if adhoc enum type will be `(.Fit|.Fill)` i.e.
>     textual representation if the declared type. As I understand this also
>     could not be a solution.. I.e. for example
>     `(Int,String,(.Fit|.Fill))->String`
>     >>>
>     >>> From other point of view, adding such type to typesystem will add
>     some consistence : you can create a function that don't need definition
>     of separate structure type(tuple will be used) and don't need separate
>     enum type(ad-hoc enum will be used). I.e. all data the function needs
>     to process could be described in function definition. Today we need to
>     use ugly Bool flags in case we want to achieve the same target.
>     >>>
>     >>>>
>     >>>> Now if enum cases were equivalent if they had the same name (like how
>     >>>> "Int" means the same thing no matter what tuple or generic type it is
>     >>>> used in), we'd have a good foundation for a self-describing structural
>     >>>> type. But this isn't how the existing named enum types work. Why would
>     >>>> it be a good idea to make anonymous enum cases interchangeable by
>     name?
>     >>>> Properties on different types aren't interchangeable, even if they
>     have
>     >>>> the same type. In fact, no type member that I am aware of is
>     >>>> interchangeable solely on the basis of name. An
>     "ArtistAction.Draw" and
>     >>>> "CowboyAction.Draw" might have the same name, but they mean completely
>     >>>> different things.
>     >>>
>     >>> I don't think they should be 'interchangeable by name', but just
>     like tuples if you defined adhoc enum with exactly the same cases as
>     ad-hoc enum in function parameters - then they are of the same type.
>     >>>
>     >>> I.e. :
>     >>>
>     >>> func foo(option: (.fit|.fill)) {..}
>     >>>
>     >>> foo(.fit) // .fit is of type  (.fit|.fill) from definition
>     >>>
>     >>> let e : (.fit|.fill) = .fit
>     >>> foo(e) // e is of (.fit|.fill) type, equal to definition
>     >>>
>     >>> but
>     >>>
>     >>> func foo2(option: (.fit|.fill|.other)) {..}
>     >>>
>     >>> foo2(.fit) // ok, here .fit is of (.fit|.fill|.other) type
>     >>> foo2(e) --> Error, e is not of type (.fit|.fill|.other)
>     >>>
>     >>>>
>     >>>> Finally, I have to ask: if you are updating your anonymous enum in
>     >>>> multiple places, how much effort have you actually saved over a
>     one-line
>     >>>> enum definition? In fact, tuples are a great example of this: best
>     >>>> practices usually state that they are good for ad-hoc destructuring,
>     >>>> such as retrieving multiple return values from a function or pattern
>     >>>> matching across several values at once, but structs are better
>     used for
>     >>>> almost everything else, since they carry semantic meaning that tuples
>     >>>> don't.
>     >>>>
>     >>>
>     >>> Just the same pros and cons for ad-hoc enums vs enum declaration as
>     for tuples vs struct declaration. Yes can use it with care and you can
>     use it in wrong way.
>     >>>
>     >>> Btw, I feel like this could be very handy to return adhoc enum:
>     >>>
>     >>> func something() -> (.one|.two|.three) {...}
>     >>>
>     >>>> I hope that clarifies my thoughts on the matter.
>     >>>>
>     >>>> Best, Austin
>     >>>>
>     >>>>
>     >>>>> On Jun 1, 2016, at 12:36 AM, Vladimir.S <svabox at gmail.com
>     <mailto:svabox at gmail.com>> wrote:
>     >>>>>
>     >>>>> On 01.06.2016 9:55, Austin Zheng via swift-evolution wrote:
>     >>>>>> Maybe it's overkill. My personal opinion is that breaking the
>     >>>>>> symmetry of the language like this (are there any other types of
>     >>>>>> function arguments that cannot be passed as either variable values
>     >>>>>> or literals?) is too much a price to pay. Your library thinks it's
>     >>>>>> being clever and vends its functions as taking anonymous enum flags,
>     >>>>>> and now there are a bunch of things I can't do with those functions
>     >>>>>> anymore.
>     >>>>>>
>     >>>>>> A regular enum can be declared in one line anyways:
>     >>>>>>
>     >>>>>> enum ScaleCropMode { case Fit, Fill }
>     >>>>>
>     >>>>> Why do we have tuples? Struct could be defined by one line `struct
>     >>>>> SomeValue { var x = 0, y = 0 }` ;-) I.e. from my point of view
>     >>>>> developer should decide what he/she wants to use: ad-hoc enum or
>     >>>>> defined enum type *exactly* as now he/she can decide to use the same
>     >>>>> tuples in multiply functions instead of one defined struct type.
>     >>>>>
>     >>>>> I replied regarding the variable on other message. (In short: I think
>     >>>>> of the same principle as for tuples: you can declare variable `let e:
>     >>>>> (.fill | .fit) = .fill` and use it)
>     >>>>>
>     >>>>>>
>     >>>>>> Austin
>     >>>>>>
>     >>>>>>> On May 31, 2016, at 11:44 PM, Charles Constant
>     >>>>>>> <charles at charlesism.com <mailto:charles at charlesism.com>
>     <mailto:charles at charlesism.com <mailto:charles at charlesism.com>>> wrote:
>     >>>>>>>
>     >>>>>>>> It breaks the ability to pass in a variable containing the
>     >>>>>>>> desired
>     >>>>>>> value, rather than the literal value itself.
>     >>>>>>>
>     >>>>>>> Maybe that's appropriate? If the caller is not passing in a
>     >>>>>>> hardcoded enum case, then that enum is probably general enough
>     >>>>>>> that it warrants a normal enum. But there are also situations
>     >>>>>>> where the same function is called from several files in the same
>     >>>>>>> code-base with different flags. Those are situations where it
>     >>>>>>> feels like overkill to clutter up my codebase with separate enums,
>     >>>>>>> only used by a single function.
>     >>>>>>>
>     >>>>>>>
>     >>>>>>>
>     >>>>>>>
>     >>>>>>>
>     >>>>>>> On Tue, May 31, 2016 at 9:24 PM, Austin Zheng via swift-evolution
>     >>>>>>> <swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>     <mailto:swift-evolution at swift.org <mailto:swift-evolution at swift.org>>>
>     >>>>>>> wrote:
>     >>>>>>>
>     >>>>>>> I admire the desire of this proposal to increase the readability
>     >>>>>>> of code. I'm -1 to the proposal itself, though:
>     >>>>>>>
>     >>>>>>> - It breaks the ability to pass in a variable containing the
>     >>>>>>> desired value, rather than the literal value itself. (Unless you
>     >>>>>>> actually want a not-so-anonymous enum type whose definition
>     >>>>>>> happens to live in a function signature rather than somewhere
>     >>>>>>> you'd usually expect a type definition to live.) - It breaks the
>     >>>>>>> ability to store a reference to the function in a variable of
>     >>>>>>> function type (ditto). - Almost every time I've wanted to use one
>     >>>>>>> of these "anonymous enums" in my code, I've ended up needing to
>     >>>>>>> use that same enum elsewhere. In my experience, 'lightweight
>     >>>>>>> enums' don't end up saving much time compared to a full-fledged
>     >>>>>>> one.
>     >>>>>>>
>     >>>>>>> Like Brent said, I have to say no to any proposal that tries to
>     >>>>>>> make enums synonyms for numerical values. What happens if you
>     >>>>>>> rearrange your anonymous enum cases between library versions? Do
>     >>>>>>> you somehow store an opaque case-to-UInt8 table somewhere for
>     >>>>>>> every anonymous enum you define for resilience? What happens when
>     >>>>>>> people start bringing back terrible C patterns, like doing
>     >>>>>>> arithmetic or bitwise ops on the underlying case values? At least
>     >>>>>>> you have to try pretty hard as it is to abuse Swift's enums.
>     >>>>>>>
>     >>>>>>> Austin
>     >>>>>>>
>     >>>>>>> On Tue, May 31, 2016 at 8:25 PM, Brent Royal-Gordon via
>  

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